Examples of superabsorbent polymer may include a crosslinked partially neutralized polymer, generally referred to as superabsorbent polymer hydrogel, including crosslinked partially neutralized polyacrylic acids or crosslinked starch-acrylic acid graft polymers, that are capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of superabsorbent polymer hydrogel, and of retaining the aqueous liquids under a certain pressure in accordance with the general definition of superabsorbent polymer.
The superabsorbent polymer hydrogel may be formed into particles, generally referred to as particulate superabsorbent polymer, wherein the particulate superabsorbent polymer may be post-treated with surface crosslinking, and/or other surface treatment to form particulate superabsorbent polymer compositions. The acronym SAP may be used herein in place of superabsorbent polymer, particulate superabsorbent polymer, superabsorbent polymer composition, particulate superabsorbent polymer compositions, or variations thereof. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCR, New York, 1998.
Commercially available superabsorbent polymer include crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are partially neutralized with a sodium hydroxide solution or a potassium hydroxide solution, hence the term “partially neutralized”.
A primary use of particulate superabsorbent polymer is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. For fit, comfort, and aesthetic reasons, and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the fluff fiber in these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more particulate superabsorbent polymer content is being used in these sanitary articles.
Color is an important property of superabsorbent polymers wherein it is imperative to minimize the yellowness of the product. Widespread consumer preference for whiter superabsorbent polymer drives manufacturers to pursue more ways to accomplish this. A yellow-white product is undesirable. Countless studies suggest that consumers clearly favor a blue-white over a yellow-white color. The former is perceived to be whiter, i.e., “fresh,” “new,” and “clean,” while the latter is judged to be “old,” “faded,” and “dirty.” Fluorescent additives can be used to enhance the appearance of the color of the polymer, causing a “whitening” effect, making materials look less yellow by increasing the overall amount of blue light reflected/emitted. In particular, in daylight, optical brighteners can thus compensate for the esthetically undesirable yellowish cast found in otherwise white (or colorless) polymers such as superabsorbent polymers.
US 2011/0303872 discloses water-absorbing polymer particles with improved color stability wherein the water-absorbing polymer particles comprise at least one optical brightener wherein the optical brightener is physically mixed with the superabsorbent polymer and/or is coated onto the superabsorbent polymer surface. In particular, US 2011/0303872 discloses that the optical brightener may be added before, during, or after the steps of i) polymerizing the polymer, ii) comminuting the polymer gel, iii) drying the polymer gel, iv) grinding and classifying the polymer gel, and optionally surface crosslinking the polymer particles. This method is still inadequate as to the bonding of the optical brightener to the superabsorbent polymer.
The present invention relates to superabsorbent copolymer comprising a reaction product of a first monomer and a reactive optical brightener comonomer, or methods to make such a superabsorbent copolymer wherein the reactive optical brightener reacts with the first monomer so as to be incorporated, directly into the polymer backbone or appended by reacting with groups of the monomer and/or the polymer, into the superabsorbent copolymer network.